Professor of Radiology
Professor of Radiology
Gasthuisberg University Hospital
Katholieke Universiteit Leuven
E: [email protected]
While, in recent years, spotlights in cardiac imaging were mainly focused on the fast progress in multidetector-row computed tomography technology, magnetic resonance imaging (MRI), in contrast, has slowly matured over the last two decades toward a fully integrated clinical imaging modality, offering the clinician valuable and often unique information for patient treatment and follow-up. In an era of fast-rising medical costs, evidence-based choice of appropriate medical imaging will become of utmost importance. This will necessitate a shift in thinking from technical efficacy and diagnostic accuracy assessment toward evaluation of the impact of imaging techniques on therapeutic decisions, patient outcome and cost- effectiveness; in other words, what test do we need to diagnose and treat a patient in the best possible way and for a reasonable (ie, payable) price? Though this important issue is beyond the scope of this review paper, the discussion about the current role of MRI in cardiovascular diseases should be regarded against this background. If we focus on the evolution of cardiac MRI since its introduction in the early 1980s, it can be said that this technique has evolved from a time-consuming, static technique, toward an almost realtime, dynamic imaging modality, competing with the non-invasive reference technique (ie, echocardiography). Within an acceptable imaging time (ie, 30–45 minutes), accurate information can be obtained on cardiac and pericardial and great vessel anatomy, ventricular and valvular function, myocardial perfusion patterns, flow quantification and tissue characterisation (eg, assessment of myocardial necrosis, myocardial scarring, pericardial inflammation). This comprehensive approach in a completely noninvasive manner is definitely the strongest point in favour of MRI.
From a practical point of view, cardiac MRI studies are usually performed on 1.5 tesla (1.5T) scanners. Imaging on 3.0T scanners is promising but yet not mature for more generalised use. Patients are installed in supine position and are asked during the exam to hold their breath repetitively. If they are unable to cooperate, respiratory triggering by use of a navigator or free-breathing real-time data acquisition are helpful alternatives. ECG triggering is a necessary requirement to obtain sharp images. Use of multichannel surface coils not only helps in optimising signal-to-noise ratios but also allows use of parallel imaging, which has been shown to be very beneficial in cardiac imaging. Speaking in a simplified way, two types of cardiac MR images are obtained: dark-blood and bright-blood images. Dark-blood images are obtained with spin-echo MRI sequences and provide morphologic information. Bright-blood images are gradient-echo-based and are used for dynamic (functional) imaging, myocardial first-pass perfusion imaging, infarct and viability imaging, and coronary artery imaging. Usually, cardiac MRI is performed along the cardiac axes (ie, short, vertical and horizontal long-axis), and a combination of imaging in different planes increases diagnostic accuracy.
For functional cardiac imaging, a volumetric, multislice approach is used, offering accurate and reproducible data on global and regional function of both ventricles. Moreover, cine MRI can be used to evaluate motion of valve leaflets, and to visualise valve regurgitation, as well as stenosis. Tagging techniques can be applied to quantify the myocardial strain (or deformation) (see Figure 1). Velocity- encoded cine MRI using the phase information allows for quantification of flow volumes and velocities, valvular regurgitation and pressure gradients and for calculation of valve orifices. Paramagnetic contrast agents are used to evaluate myocardial perfusion, to increase contrast between pathological (eg, necrotic, scar or inflamed tissues) and normal tissue. Submillimetre- spatial-resolution three-dimensional techniques are available to study the coronary arteries. Although still hampered by long acquisition times, the devastating effect of calcium in atherosclerotic plaques when assessing stenosis severity that currently hampers multidetector-row computed tomography is much less an issue with MRI.
From a clinical point of view, the question arises how MRI compares with other “competing” cardiac imaging techniques. In an interesting Consensus Panel Report paper by the Society for Cardiovascular Magnetic Resonance and Working Group on cardiovascular MR of the European Society of Cardiology, the usefulness of MRI in specific cardiac diseases was classified according to the degree of clinically relevant information provided (Class I–II–III, Class investigational). A Class I indication (ie, may be used as first-line imaging technique) was given to: the diagnosis and follow-up of congenital heart disease in adults; follow-up of aortic dissection and other abnormalities of the great vessels; assessment of global ventricular (right and left) function and mass; detection and assessment of acute and chronic myocardial infarction and myocardial viability; detection and characterisation of cardiac and pericardiac tumours; arrhythmogenic right ventricular dysplasia; iron overload cardiomyopathy; dilated cardiomyopathy; apical forms of hypertrophic cardiomyopathy; and quantification of valvular regurgitation and repercussion of valvular heart disease on cardiac chamber anatomy and function. For the study of the above and other (Class) indications, a combination of MRI sequences, fine-tuned to the clinical question, is usually chosen. For instance, in patients with an acute myocardial infarction, the main interest is:
- To evaluate the presence, location and extent of myocardial necrosis, and the concomitant
- presence of microvascular obstruction.
- To evaluate the impact on global and regional function.
- To determine the extent of area at risk.
- To rule out complications.
This information is obtained using contrast-enhanced inversion-recovery MRI with late imaging (a,d), dynamic cine MRI along the cardiac axes (b) and T2-weighted short-tau inversion-recovery MRI (c) (see Figure 2). Knowledge about specific enhancement patterns in different types of myocardial diseases has been shown to be highly beneficial in differentiating ischaemic from different types of nonischaemic mycardial diseases (see Figure 3). In patients with angina-like chest pain, presence of functionally significant coronary stenoses can be detected using pharmacological (dobutamine–adenosine/dipyridamole) stress MRI protocols. In patients presenting with diastolic heart failure, differentiation between restrictive cardiomyopathy and constrictive pericarditis is crucial to determine the ideal therapeutic strategies. This can be achieved by combining morphological assessment of the pericardium and heart with assessment of ventricular inflow patterns and ventricular coupling. MRI has been shown to be the preferred technique to follow up certain types of congenital heart disease, such as postoperative patients with Fallot’s tetralogy and transposition of the great arteries (see Figure 4). Also, in patients with suspected cardiac masses and tumours, cardiac MRI can be considered as first-line.
MRI should be considered as an important player in the domain of cardiovascular imaging. It combines accuracy and versatility in a reliable way, and has the potential to become the preferred imaging modality to assess a large variety of cardiovascular diseases.